Mr Trump said he would start the effort to replace Kennedy "immediately" and would pick from a list of 25 names that he updated last year. Senate Majority Leader Mitch McConnell declared that the Senate "will vote to confirm Justice Kennedy's successor this fall."

Republicans also have a chance to make judicial nominees a top campaign issue, which could help motivate conservatives and evangelicals to vote in November. The playbook worked in 2016, when Republicans rallied around McConnell's successful block of then-President Barack Obama's nominee to the court, Merrick Garland.

But while Republicans are aiming for speedy action, Democrats quickly argued that any decision should be put on hold until after midterm elections, citing McConnell's 2016 moves. Senate Minority Leader Chuck Schumer, D-N.Y., said it would be the "height of hypocrisy" to vote sooner.

Americans are about to see if @SenateMajLdr & @SenateGOP will heed the McConnell Rule or rashly proceed to confirm a nominee.

The future of our democracy is at stake. The people deserve a voice in a decision that could radically alter the course of American justice for decades. https://t.co/wIvTSM9gb0

He said the voices of millions of Americans heading to the polls this fall "deserve to be heard."

McConnell refused to consider Garland because it was a presidential election year. He said the seat should be left open for the next president to fill.

Trump: Choosing Supreme Court judge one of most important events

Mr Trump took a moment on Wednesday to explain the significance of a Supreme Court vacancy to visiting Portuguese President Marcelo Rebelo de Sousa.

U.S. President Trump speaks to reporters during meeting with Portugal's President Rebelo de Sousa at the White House in WashingtonCredit:
Reuters

The US president greeted Rebelo de Sousa in the Oval Office and then turned to the cameras to offer his reaction to Kennedy's retirement.

"In our country, the selection of a justice of the United States Supreme Court is considered, I think we can all say, one of the most important events," Mr Trump told Rebelo de Sousa, adding that the court can "swing different ways" depending on its composition.

"Some people think outside of, obviously war and peace, it's the most important thing that you could have," Mr Trump said.

Mr Rebelo de Sousa concurred, noting that he was also a constitutional law professor. "I just heard the news and I imagine how important it is for you," he said, pivoting to Portugal's role as the first neutral country to recognise America's independence.

Trump jokes about Ronaldo

As well as the supreme court, Mr Trump and Mr Rebelo de Sousa discussed the World Cup in Russia.

The Portuguese president suggested "if you ever go to Russia during the championship, don't forget Portugal is still there and wanting to win the championship."

Rebelo de Sousa said he had recently met with Putin, who asked him to extend his greetings to Trump and was "expecting your visit."

But the Portuguese leader also wanted to make sure Mr Trump knew that his country has Ronaldo, the best football player in the world. "I'm sure your son knows it," he told Mr Trump. The president said his son, Barron, an avid football player, "knows all about it."

With the specter of a Supreme Court fight and tough mid-term elections ahead, Mr Trump tossed out his own hypothetical. Would Ronaldo, the celebrity football star leading Portugal's World Cup team, "ever run for president against you?"

"He wouldn't win. You know he won't," Mr Trump assured him.

Mr Rebelo de Sousa wasn't about to go there. "You know something. I must tell you, Portugal is not just the United States."